The present invention relates to implantable cardiac pacers and more particularly to such a pacer which is hermetically sealed.
While pacers of a construction in which both pulse-generating circuitry and batteries are encased in plastic resin such as epoxy have been used successfully for many years, a continuing concern has been the effect moisture from the patient's body may have on circuit components it it penetrates the enclosure over time. Accordingly, attempts have been made from time to time to construct a hermetically sealed pacer, that is, a pacer in which the pulse-generating circuitry and the batteries powering the circuitry are contained within a metallic case which is sealed so as to be effectively impervious with respect to either gases or liquids. Signals into and out of the circuitry are coupled through the case by means of feed-through terminals of various types known in the art, e.g. those employing glass-to-metal seals and the like.
A persistent problem in arriving at a satisfactory hermetically sealed pacer construction has been the management of the atmosphere within the hermetically sealed case. While the electronic components employed for generating pulses may typically be considered to be essentially inert, the batteries are not. The most prevalent type of battery for powering cardiac pacemakers, i.e. due to its relatively long life and stability, is that employing so-called mercury cells. The electro-chemical reaction taking place in these cells generates hydrogen. The cells themselves are typically constructed so as to vent this hydrogen in a predetermined and controllable manner. In pacers constructed with a resin encasement or encapsulation, the hydrogen can permeate the casing material and is thus given off by the pacer and eventually absorbed by the fluids in the patient's body. The rate of discharge of hydrogen is sufficiently low that dangerously high pressures are not encountered. In a hermetically sealed pacer, however, the internal pressure inside the hermetically sealed case can continue to grow until it may reach levels which will bulge the case, changing its buoyancy, and possibly damage electronic components. The present invention involves the management of the atmosphere within a hermetically sealed cardiac pacer in such a manner that dangerous pressures are avoided.
Among the several objects of the present invention may be noted the provision of a novel, hermetically sealed implantable cardiac pacer and method of fabrication; the provision of such a pacer which is energizable by mercury batteries; the provision of such a pacer which is highly reliable; the provision of such a pacer in which electronic components are not subjected to abnormally high temperatures; and the provision of such a pacer which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.